1. Field of the Invention
The present invention related to a traction control of vehicles such as automobiles, and more particularly, to a traction control system of vehicles by which the traction. i.e. driving or braking, of a vehicle is controlled according to a combination of a feedback control and a feedforward control.
2. Description of the Prior Art
In Japanese Patent Laid-open Publications 7-127492 and 7-174042, it is described to control the driving torque of each of the driving wheels by a feedback control based upon a target driving torque calculated therefor based upon running conditions of the vehicle.
In Japanese Patent Laid-open Publications 8-310366 and 9-99826, it is described to control the braking force of each of the wheels by a feedback control such that a target yaw moment and a target deceleration to be generated in a vehicle are calculated based upon operating conditions thereof, a target braking force to be applied to each of the wheels is calculated based upon the target yaw rate and the target deceleration, a target slip ratio to occur at each of the wheels is calculated based upon the target braking force, and the brake pressure is controlled to let each of the wheels cause a slip ratio coinciding with the target slip ratio.
In general, a feedback control is much more reliable than a feedforward control in maintaining a parameter for control within a desired range, while a feedback control is less effective than a feedforward control in quickly controlling a parameter for control to a desired target value. From another point of view, a feedback control is superior to a feedforward control when a parameter to be controlled is not calculatable at high precision by theory, while, as a matter of course, a feedforward control is more effectively adaptable when a parameter to be controlled is calculatable at higher precision
In the art of vehicle control, the parameters within the vehicles are nowadays generally calculatable at high precision by theories and computers, while in the traction control there still remains an obstruction for a high precision theoretical calculation in relation to the frictional contact between the wheels and the ground surface.
In view of the above, it is contemplated that the traction control of vehicles is further improved by incorporating a feedforward control into a traction control system based upon a feedback control, wherein the feedback control is essential for a final stage of control relying upon the frictional contact between the wheel tires and the ground surface not always theoretically correctly predictable.
Therefore, it is a primary object of the present invention to provide an improved traction control system of vehicles in which a feedback control is favorably combined with a feedforward control.
According to the present invention, the above-mentioned primary object is accomplished by a traction control system of a vehicle having a vehicle body, a plurality of wheels including at least one pair of driving wheels for suspending the vehicle body, an engine, a transmission for transmitting an output power of the engine to the driving wheels, a brake system for selectively braking rotation of at least the pair of driving wheels, and a steering system for steering the vehicle, the traction control system comprising:
means for calculating a target traction torque of each of the pair of driving wheels based upon operating conditions of the vehicle;
means for calculating a target slip ratio of each of the pair of driving wheels based upon the target traction torque calculated therefor, and
means for controlling the engine and the brake system such that actual slip ratio of each of the pair of driving wheels coincides with the target slip ratio calculated therefor according to a feedback control, with a partial feedforward control of the engine and the brake system based upon the target traction torque.
By the traction control system of a vehicle being constructed as described above, the engine and the brake system thereof are controlled to accomplish a slip ratio at each of the pair of driving wheels theoretically calculated for obtaining a desired operation performance of the vehicle according to such a feedback manner in reference to the actual ground surface condition that, when the actual ground surface condition is different from its estimation, the theoretical calculation is automatically modified to reflect the difference in estimation thereon through corresponding changes of parameters for the theoretical calculation, while the engine and the brake system are directly modified according to the result of the theoretical calculation for the slip ratio at each of the pair of driving wheels according to the feedforward manner. Therefore, it is accomplished that the quickness of the response of control of the traction control system of a vehicle is improved, while ensuring that the vehicle is controlled to perform without much departing from a desired operating condition even when the ground surface condition changes beyond a reasonable estimation.
In the above-mentioned traction control system, the engine and brake system control means comprises: means for calculating a feedback traction torque of each of the pair of driving wheels which will cancel a difference between the calculated slip ratio and the actual slip ratio of each of the pair of driving wheels; means for calculating a sum of a value of the calculated target traction torque weighted with a feedforward control weighting factor and a value of the feedback traction torque weighted with a feedback control weighting factor with respect to each of the pair of driving wheels; and means for controlling the engine to generate an output torque corresponding to the weighted sum of the target traction torque and the feedback traction torque with respect to each of the pair of driving wheels when the weighted sum is positive, while controlling the brake system to generate a braking torque at each of the pair of driving wheels corresponding to the weighted sum of the target traction torque and the feedback traction torque when said weighted sum is negative.
The feedforward control weighting factor and the feedback control weighting factor may be complementary to one another.
Further, the means for calculating the weighted sum of the feedforward traction torque and the feedback traction torque may include means for temporarily lowering the feedforward control weighting factor when the vehicle is running with an engine braking.
The means for calculating the feedback traction torque may comprise means for temporarily modifying the calculation of the feedback traction not to be dependent upon the target traction torque but to be dependent substantially upon vehicle speed when the vehicle is running with an engine braking.
Further, the means for calculating the feedback traction torque may also comprise means for temporarily modifying the calculation of the feedback traction torque not to be dependent upon the target traction torque but to be dependent substantially upon vehicle speed when a target slip ratio calculated based upon the target traction torque is larger than a predetermined threshold value.
The target traction torque calculation means may further comprise means for calculating a yaw moment of the vehicle based upon steering angle of the steering system and vehicle speed; means for equating the calculated yaw moment to a corresponding balance between a pair of traction forces of the pair of driving wheels; means for equating a longitudinal acceleration of the vehicle to a corresponding sum of a pair of traction forces of the pair of driving wheels; and means for deducing the values of the pair of traction forces from the above two relationships with regard to the yaw moment and the longitudinal acceleration.
In this case, the target traction torque calculation means may further comprises: means for calculating a friction circle of each of the pair of driving wheels; and means for trimming each of the deduced values of the pair of traction forces to be within each corresponding one of the friction circles calculated therefor.
In this case, further, the friction circle calculation means may comprise: means for calculating each of longitudinal forces acting at the pair of driving wheels as a sum of a corresponding share of a force accelerating the vehicle at a longitudinal acceleration thereof, a force exerted thereto by the brake system, and a force for acceleration of rotation thereof against the inertia thereof; means for calculating a longitudinal force acting at each of the pair of driving wheels as a sum of a corresponding share of a longitudinal force corresponding to an output torque of the torque converter calculated based upon rotation speed of the engine, a force exerted thereto by the brake system, and a force for acceleration of rotation thereof against the inertia thereof; means for calculating each of ground contact forces acting at all of the wheels based upon longitudinal acceleration of the vehicle and lateral acceleration of the vehicle; means for calculating each of ground contact forces acting at all of the wheels based upon longitudinal acceleration of the vehicle and lateral acceleration of the vehicle; means for calculating slip angle of the vehicle body based upon lateral acceleration of the vehicle, vehicle speed and yaw rate; means for calculating each of cornering forces of the pair of driving wheels based upon vehicle speed, a time based differential of the calculated slip angle of the vehicle, yaw rate and a time based differential thereof of the vehicle, and the ground contact forces calculated for all of the wheels; and means for calculating a root of a sum of a square of the longitudinal force and a square of the cornering force with respect to each of the pair of driving wheels.
Further, the means for calculating the longitudinal forces acting at each of the pair of driving wheels may cancel the longitudinal force based upon the output torque of the torque converter when the transmission is being shifted between speed stages.
Further, the friction circle calculation means may further comprise means for judging if wheel speed of each of the pair of driving wheels is larger than vehicle speed beyond a first predetermined difference over a predetermined duration; and means for decreasing the radius of the friction circle of the corresponding wheel when the answer of the judgment is yes.
Further, the friction circle calculation means may further comprise means for judging if wheel speed of each of the pair of driving wheels is larger than vehicle speed beyond a second predetermined difference larger than the first predetermined difference; and means for decreasing the radius of the friction circle of the corresponding wheel when the answer of the judgment is yes.
In the above construction, the friction circle calculation means may further comprise means for regaining the radius of the friction circle of the corresponding wheel when the answer of the judgment is no up to a standard friction circle radius.